Smad-interacting protein 1 (Zfhx1b, Sip1) and Zfhx1a (deltaEF1) are transcriptional repressors belonging to the family of two-handed zinc finger/homeodomain proteins. Both of the proteins bind to a bipartite CACCT/CACCTG DNA sequence, but only Sip 1 can interact with activated SMAD proteins in vitro. To gain insight into their developmental roles, we investigated the mRNA expression patterns of both genes in developing mouse and chick limbs by in situ hybridization. To improve the resolution of the expression mapping we have used a dual in situ hybridization (DISH) technique allowing for a detection of expression of two different mRNAs on the same tissue section. Using DISH we could demonstrate that both genes were expressed in distinct non-overlapping patterns in developing limbs of both species. Zfhx1a was expressed in a cell population immediately adjacent to cartilage anlagen as well as in developing tendons. Zfhx1b, in contrast, was present in a broad area around developing tendon and partially overlapping with the expression of genes associated with myogenic differentiation.